The invention relates to improvements in electric motors and in particular, to an improved brushless repulsion type motor. Conventional repulsion motors are typically constructed with a single phase stator and a DC rotor having an armature winding connected to a commutator.
Diametrically opposed carbon brushes riding on the commutator are shorted together, but are not directly connected to the AC power line. When AC power is applied to the stator winding, currents are induced in the armature to create the rotor field. Important advantages possessed by the repulsion motor are the relatively high value of the starting torque with comparatively low starting current, the ability to sustain high starting torques for long periods of time, such as may exist under conditions of high inertial load, and an adaptability to wide range speed control.
The speed torque curve of a repulsion motor is similar to that of universal series motors or series type DC motors. The no-load speed of the repulsion motor can be many times higher than the synchronous speed. A problem with the conventional repulsion motor is that the brushes and commutator wear out quickly due to arcing and heat generated by the brushes in contact with the commutator. As a result, basic repulsion motors are not commonly used today because of the brush wear problem.
Other motor types have been designed to minimize these problems. For example, a repulsion start, induction run motor is designed with a squirrel cage rotor embedded in the wound armature. Mechanical means are used to lift the brushes from the commutator when the rotor speed reaches a predetermined value, and the motor then runs as an induction motor. This is done to develop a very high starting torque for the induction motor.
Another motor is disclosed in U.S. Pat. No. 5,424,625, incorporated by reference herein. In accordance with that disclosure, electronic switching means is carried on the rotating armature to short individual coils at appropriate times in a cycle of rotation to eliminate the need for brush and commutator elements. Specifically, an electronic switch circuit is provided for replacing the switch and current carrying function of one pair of oppositely disposed commutator segments or bars. Electrical power needed to energize the electronic switching means and any related control circuitry on the armature is produced on the armature by induction from the stator field. The control electronics on the armature include circuitry to sense an enabling signal from stationary signaling means mounted on the stator in order to control the actuation of the electronic switches. Control circuitry is operative when a coil is at a predetermined angular position, relative to the stator. Each switch shorts the ends of an associated coil together. The result of this short is essentially the same as that achieved in the prior art by a pair of opposed shorted brushes.
However, the armature and induction field within such motor housing produce heat which is not easily dissipated and the temperature in the motor rises. Such elevated temperature reduces the power capability, reliability, and life of electronic switches and other components. It is also very difficult to replace or repair the electronic components within the motor housing. This involves complete disassembly of the motor, which is both time consuming and expensive.